Obstructive sleep apnea (OSA) is associated with an increased risk for cardiovascular events. Whether the mechanism for this association is directly attributable to pro-atherosclerotic effects of OSA or primarily related to obesity is not known. Obesity is common in patients with OSA, and is itself associated with an inflammatory state and insulin resistance. In individuals with both obesity and OSA, the repetitive hypoxia associated with OSA may promote inflammation and insulin resistance. Our primary aim is to test the hypothesis that OSA plays a primary role in the inflammatory state of patients with obesity and moderate-severe OSA, and that therefore the elimination of apnea by continuous positive airway pressure (CPAP) therapy in these patients will more greatly reduce inflammation (measured by C-reactive protein or CRP) than weight loss alone, and that combining these two therapies will have additive effects on reducing inflammation.; Our second aim is to test the hypothesis that OSA and obesity independently contribute to the insulin resistant state in these patients, and thus combined weight loss and CPAP therapy will have greater effects on lowering insulin resistance than either therapy alone. Our third aim is to test the hypothesis that OSA and obesity contribute independently to vascular endothelial dysfunction, through their effects on inflammation and insulin resistance, and that therefore combining weight loss and CPAP therapy will improve endothelial function more than either therapy alone. To address these aims, we plan to randomize 201 subjects with obesity and moderate-severe OSA, and baseline CRP>1.0 mg/L to 1) weight loss theYapy alone; 2) CPAP therapy alone; or 3) weight loss plus CPAP therapy for 6 months. We will measure CRP, insulin resistance (area under the curve of a glucose tolerance test), and endothelial function (by brachial artery flow studies) at baseline, and weeks 6, 12, and 24. We propose that this study design will provide the best way to separate the independent effects of obesity and OSA on cardiovascular risk.